Printed wiring boards (also known as printed circuit boards) are generally laminated materials comprised of two or more plates or foils of copper, which are separated from each other by a layer of nonconducting material. Although copper is generally used as the electroplating metal in printed wiring boards, those skilled in the art will recognize that other metals such as nickel, gold, palladium, silver and the like can also be electroplated. The nonconducting layer or layers are preferably an organic material such as an epoxy resin impregnated with glass fibers, but may also comprise thermosetting resins, thermoplastic resins, and mixtures thereof, with or without reinforcing materials such as fiberglass and fillers.
In many printed wiring board designs, the electrical pathway or pattern requires a connection between the separated copper layers at certain points in the pattern. Through holes are formed in printed circuit boards in order to establish connection between the circuit layers at certain points in the board to produce the desired electrical pattern. This is usually accomplished by drilling holes at the desired locations through the copper layers and the non-conducting layers, and then connecting the separate circuit layers by metallizing the through holes (i.e., coating the inner surface of the drilled or punched through hole with a conductive metal). The hole diameters of PCBs generally range from about 0.15 mm to about 10.0 mm, more typically from about 0.3 mm to about 1.0 mm.
While electroplating is a desirable method of depositing copper and other conductive metals on a surface, electroplating cannot be used to coat a nonconductive surface, such as an untreated through hole. It has thus been necessary to treat the through hole with a conductive material to make it amenable to electroplating. One process for making the through hole bores electrically conductive, is to physically coat them with a conductive film. The coated through holes are conductive enough to electroplate, but typically are not conductive and sturdy enough to form the permanent electrical connection between the circuit layers at either end of the through hole. The coated through holes are then electroplated to provide a permanent connection. Electroplating lowers the resistance of the through hole bore to a negligible level, which will not consume an appreciable amount of power or alter circuit characteristics. One advantageous way of preparing the through hole walls for electroplating utilizes a liquid carbon dispersion. The steps of this process are discussed briefly below.
First, surfaces of the through holes are drilled and deburred. In the case of multilayer printed circuit boards, it may also be desirable to subject the boards to a desmear or etchback operation to clean the inner copper interfacing surfaces of the through holes. Such methods are well known to those skilled in the art.
Then, the printed wiring board is preferably subjected to a precleaning process in order to place the printed wiring board in condition for receiving the liquid carbon black dispersion. After the application of the cleaner, the PWB is rinsed in water to remove excess cleaner from the board and then contacted with a conditioner solution. The conditioner solution is used to ensure that substantially all of the hole wall glass/epoxy surfaces are properly prepared to accept a continuous layer of the subsequent carbon black particles. See for example U.S. Pat. No. 4,634,691, to Lindsey, the subject matter of which is herein incorporate by reference in its entirety, which describes a suitable conditioner solution.
The liquid carbon dispersion is next applied to or contacted with the conditioned PWB. This dispersion contains three critical ingredients, namely, carbon black, one or more surfactants capable of dispersing the carbon and a liquid dispersing medium such as water. The preferred methods of applying the dispersion to the PCB include immersion, spraying or other methods of applying chemicals that are typically used in the printed circuit board industry. A single working bath is generally sufficient for applying this carbon dispersion; however, more than one bath may be used for rework or other purposes.
The carbon covered board is then subjected to a step where substantially all (i.e., more than about 95% by weight) of the water in the applied dispersion is removed and a dried deposit containing carbon is left in the holes and on other exposed surfaces of the nonconducting layer. To insure complete coverage of the hole walls, the procedure of immersing the board in the liquid carbon dispersion and then drying may be repeated.
The carbon (black) covered board is next optionally subjected to an additional graphite treatment yielding the deposition of a graphite layer on top of the carbon layer. In this instance, the carbon (black)-coated PWB board is preferably first contacted with a conditioner solution, which is used to promote subsequent adsorption of the dispersed graphite particles on the carbon (black) layer. After the application of this optional conditioner solution, the PWB is subsequently rinsed with water to remove excess conditioner from the board. The board may then be contacted with the liquid graphite dispersion or suspension. The board is then subjected to a step where substantially all (i.e., more than about 95% by weight) of the water in the applied dispersion is removed and a dried graphite deposit is left in the holes over the carbon (black) deposit and on other exposed surfaces of the nonconducting layer.
The steps of this process are described in more detail, for example, in U.S. Pat. No. 4,619,741, the subject matter of which is herein incorporated by reference in its entirety. Various modifications and refinements to this process are set forth in U.S. Pat. Nos. 4,622,107, 4,622,108, 4,631,117, 4,684,560, 4,718,993, 4,724,005, 4,874,477, 4,897,164, 4,964,959, 4,994,153, 5,015,339, 5,106,537, 5,110,355, 5,139,642, and 5,143,592, the subject matter of each of which is herein incorporated by reference in its entirety.
A continuing challenge in the art of carbon-based direct metallization is increasing the conductivity of the carbon coating that is deposited on the nonconductive surface to achieve faster electroplating, to allow electroplating over larger areas, and to provide other benefits.
Various methods have been suggested for increasing the conductivity of the carbon coating that is deposited. For example, U.S. Pat. No. 5,476,580 to Thorn et al., the subject matter of which is herein incorporated by reference in its entirety proposed to modify carbon (graphic) by adding surfactants or binders to the dispersion. U.S. Pat. No. 5,759,378 to Ferrier et al., the subject matter of which is herein incorporated by reference in its entirety, modifies the carbon black itself to reduce the resistivity of the carbon layer or improve the uniformity of the carbon layer on the non-conductive surface and/or the uniformity of the dispersion creating the carbon layer, increase the activity of the carbon surface toward plating, or combinations thereof.
Ferrier et al. describe various modifications to the carbon black, including treating the carbon with a dye prior to incorporating it in the dispersion composition, treating the carbon with various metals such that the metals are either absorbed onto the surface of the carbon or reduced into the surface of the carbon, and oxidation of the surface of the carbon, such as by chemical oxidation of the carbon, i.e., mixing the carbon with a solution of nitric acid for a time and at a temperature effective to appropriately oxidize the surface of the carbon. Ferrier et al. disclose that the carbon dispersion formed with the modified carbon is more uniform or the carbon forms a more uniform, more adherent, more active or less resistive coating on the non-conductive surface. These changes in the dispersion and/or the carbon coating, which resulted from the modification of the carbon itself, manifest themselves in improved coverage of the non-conductive surface with the plated metal, improved adhesion of the plated metal to the non-conductive surface, increased plating propagation rate, decreased resistance of the carbon coated non-conductive surface, or decreased plating time necessary to achieve complete coverage of the non-conductive surface with the plated metal.
However, there remains a need in the art for additional improvements in the conductivity of the carbon deposited on the non-conductive substrate.